[DungeonMaster]

The proper termonology is Reserve Margin which historically has been 20% above than peak demand. Last year, I believe that nation wide reserve margin was in the 15% range which is pushing the boundries and in some regions was actually lower than that. Utilities plan major maintenance outages around both demand projections (usually spring and fall when demand is lower) and the amount of reserve available. One of the problems with wind is that it is diffuicult to even count it among reserve margin since you can't plan ahead a year, a month or even a day on it being there when you need it.

That 15 percent reserve margin must be from peak demand, right? You are right, wind predictions are only good for a few days or hours in advance.

[Ditto]

That 15 percent reserve margin must be from peak demand, right?

Correct. But that is why electricity is different than other commodities. There is no inventory of it. The system has to be built to meet the peak and every year, the peaks keep growing. Electricity is created on demand and if demand is higher than what can be generated at any given moment we have our choice of local blackouts to reduce demand or total grid collapse. Neither are good things.